The reason why libertarianism is impracticable is as simple as this:
http://www.cpsc.gov/onsafety/2011/06/the-new-crib-standard-questions-and-answers/
It is not possible for individual consumers to make good purchasing decisions in an unregulated market. Not possible, as in impossible. Why? Individual consumers cannot be experts in everything. They do not know--and cannot reasonably be expected to know--what a safe distance between slats in a baby crib is. They do not know--and cannot be expected to know--what ingredients in food or materials in goods are carcinogenic. They do not know--and cannot be expected to know--whether any given food they buy has been grown and stored safely until the point of sale. Nor would it even be desirable for every citizen to know these things. It would require economic standstill to require every individual in a society to become an expert in every conceivable topic that could negatively affect them or their family in terms of products that they consume. There would no longer be time for work if we all had to become scientists in every conceivable scientific field.
Not only that, even if we did all become experts in every conceivable subject impacting our safety with respect to products we buy, without regulation--i.e., without social planning--we would still be at the mercy of the sellers of those products concealing relevant information about them.
Libertarianism doesn't require more than a handful of seconds of critical thought to reject.